Pulse transmission methods include various methods such as the OOK (On Off Keying) modulation scheme of transmitting information depending on whether or not there is a pulse, the PPM (Pulse Position Modulation) modulation scheme of transmitting information according to the pulse positions and the PWM (Pulse Width Modulation) modulation scheme of transmitting information according to the pulse width, and the optimal modulation scheme is adapted according to the system in which transmission is carried out.
Further, methods of transmitting information from the transmitting end to the receiving end include the parallel transmitting method of transmitting information in a plurality of transmission sequences and the serial transmitting method of transmitting information in one transmission sequence. The serial transmitting method refers to a method of time-dividing one transmission sequence and multiplexing and transmitting information of an applicable sequence per time that is time-divided. The serial transmitting method uses a smaller number of transmission sequences than the parallel transmitting method of performing transmission using a plurality of transmission sequences, and provides an advantage of simplifying the configuration of the transmitting means. Consequently, the serial transmitting method is adopted more often than the parallel transmitting method in wired communication and wireless communication in recent years.
For example, Patent Document 1 discloses a serial transmitting method using the PWM modulation scheme as a serial transmitting method using the pulse transmitting scheme. The method disclosed in Patent Document 1 refers to transmitting a plurality of combinations of data using pulse signals of a pulse width determined in advance. FIG. 1 shows the relationship between data and pulse signals in a case where the method disclosed in Patent Document 1 is used. As shown in FIG. 1, according to this transmitting method, transmission is performed by defining and arranging four pulse widths (W00, W01, W10 and W11) in the time of symbol period T, depending on the value of m, which represents a quadrature of symbol data comprised of n=2 bits, and allowing transition from H (Hi) to L (Low) only once.
FIG. 2 shows a main configuration of a demodulating apparatus used in the transmitting method disclosed in Patent Document 1. Rise detecting section 11 detects the rise of the received PWM modulated signal and reproduces a clock signal employing a clock period twice symbol period T. Then, PLL circuit 12 uses the reproduced clock signal to generate a high-speed clock signal for detecting the minimum pulse width. Counter 13 measures each pulse width of the PWM modulated signal using the generated high-speed clock signal as a reference. Data generating section 14 estimates the pulse width based on the measured value, and symbol data matching the pulse width is converted and demodulated. That is, given that the above transmitting method enables serial transmission of a plurality of bits by means of the pulse width, there is an advantage that use efficiency of the transmission sequence is high.
Further, in the above transmitting method, given that the transition from H to L takes place only once in the time of symbol period T, the symbol clock of symbol period T can be extracted from the PWM modulated signal received in the demodulating apparatus, so that a high-speed clock signal synchronized with the PWM modulated signal can be generated as a high-speed clock signal for detecting the minimum pulse width. Furthermore, the pulse width is detected by measuring each pulse width of the received PWM modulated signal using the high-speed clock signal, and symbol data matching the pulse width is demodulated.    Patent Document 1: Japanese Patent Application Laid-Open No. HEI9-36923 (page 7, FIG. 3, and page 8, FIG. 2)